The present invention relates to a method and apparatus for expressing liquor from a moving fiber batt.
Textile fibers are typically wet treated as staple or in heavy weight, nonwoven batt-like formations prior to subsequent light weight, nonwoven web formation or yarn spinning. For example, the scouring and bleaching of cotton fiber for use in the manufacture of medical and health care products is currently carried out in batch-kier processes. Some textile fibers are also stock dyed in batch processes in large dye kettles, vats, or kiers prior to carding and spinning. Other chemical treatments may at times be applied more advantageously to textile fibers in "stock" or "staple" form rather than to yarn or to fabric.
For technical and economic reasons, however, it is preferable to wash, scour, bleach, dye or otherwise treat textile fibers by continuous processes rather than by batch processes. In such continuous processes, it is frequently preferable to apply such chemical treating liquors to fibers which have been opened, carded, and/or otherwise formed into long continuous nonwoven batts weighing at least 8 oz. per square yard and typically ranging from about 16 oz. to about 48 oz. of dry fiber per square yard of batt.
In wet physical or chemical treatments such as those described above, the treatments may be applied to textile fibers that have been prepared in continuous batt form. The fibers to be treated may preferably be transported upon a series of endless belts through a series of small volume chemical processing vessels (which are relatively long and shallow, rather than deep) in order to apply a planned sequence of wet physical or chemical treatments. As the fiber (in a continuous batt-like form, supported by a series of endless conveyor belts) passes from one wet processing step to another wet processing step, it is generally desirable to reduce the percentage of total wet pickup of a treating liquor (and accordingly the weight) with respect to a dry fiber batt. After the batt passes out of the treating liquor of an impregnation vessel, the batt is passed into other processing vessels. These could include another impregnator, a rinser, an aging (reacting) chamber, a drier, or a subsequent treating liquor vessel (impregnator).
Reduction of the percentage of wet pickup to a desired process control application level between any two given processing stages may be accomplished, for example, by the use of paired squeeze rolls, or by the use of a vacuum slot or plenum device. However, a vacuum slot requires specially designed equipment to provide a suitable vacuum, and, for nonwoven batts, a specially designed conveyor belt or perforated drum is necessary to carry the batt over the vacuum slot or the plenum.
An important commercial interest is concerned with improved devices and methods for employing paired high expression squeeze or nip rolls to squeeze excess treating liquor from the batt. To obtain high expression efficiency, it is sometimes impractical to pass the impregnator or rinser primary conveyor belt along with the superimposed batt through the nip between the high expression squeeze rolls. Especially in the case of fibrous batts possessing highly competitive capillary systems relative to the capillary pore structure and pore volume of the supporting conveyor belt, it is not readily practical to pass both the belt and batt through the nip of the rolls.
When both the belt and the batt are passed through the nip, the conveyor belt is generally porous to permit the liquor expressed by the paired squeeze roll nip to drain through the belt. Unfortunately, the pore structure of the belt typically retains a significant amount of liquor per unit area of belt as the batt and belt pass together through the nip of the paired squeeze rolls. Then, as the batt and the belt emerge in close capillary contact with each other, downstream of the nip, the fine capillary structure of the fiber batt typically re-absorbs liquor from the coarser pore structure of the belt. Such re-absorption lowers the efficiency of the nip rolls in expressing liquor from the batt. Hence, usually it is preferred to use separate conveyor belts, one belt carrying the batt up to the input side of the nip rolls, and the second belt carrying the batt away from the nip rolls.
Whenever the batt is passed through the nip not supported on a conveyor belt, considerable ingenuity must be employed in arranging the conveyor belts and in positioning the belt turn rolls both immediately upstream and immediately downstream of the high expression squeeze rolls in order to assure smooth operational transfer of the batt. The batt must be transferred from the first belt into the nip of the squeeze rolls, and then from the squeeze rolls onto the next conveyor belt. Even though proper attention to such details can greatly improve the transfer efficiency of the batt, there remains a potentially troublesome problem.
If the liquor being expressed from the batt at the nip of the high pressure squeeze rolls is too copious, the weight of the flow of liquor will be sufficiently heavy to cause the batt to distort and rupture. Such a situation is more likely to occur with relatively heavier weight batts at higher linear rates of batt travel through the squeeze rolls. A heavier weight batt increases the volume of liquor expressed per unit length of batt and hence per unit time. Higher linear speeds of batt travel also increase the volume of liquor expressed per unit time.
Many attempts have been made to overcome the problem of batt rupture at high rates of liquor expression but these attempts have been found to be ineffective, mechanically troublesome, and/or excessively costly to employ. For example, a plurality of sets of paired nip rolls could be employed in a tandem sequence to reduce the liquor content of the batt in a series of fractional steps. However, such a deployment of a series of paired nip rolls not only adds significantly to the capital, space, and energy costs, but also adds to the number of potentially troublesome transfer points.
In view of the economic advantages gained by processing heavier area density fiber batts at higher linear speeds through paired high expression squeeze rolls, each pair positioned immediately after an impregnator or rinser, considerable effort has been expended toward the improvement of squeeze roll arrangements. In particular, considerable effort was made to adapt various conveyor belt fabric designs and various endless belt conveyance designs in an auxiliary batt transfer belt passing through the nip with the batt to provide an arrangement which satisfies process efficiency requirements. An efficiency process requires that the use of such an auxiliary batt transfer belt (a) does not interfere significantly with the efficiency of the squeeze rolls in expressing the rinsing or the treating liquors from the batt, (b) that the high volume of liquor expressed from the batt does not rupture or disrupt the uniform fiber formation of the batt, (c) that the conveyor belt track properly during the travel of the endless belt through its endless path about turn rolls and through the nip of the squeeze rolls, and (d) that the conveyor belt retains the integrity of its essential dimensional characteristics of length and width.
Many alternatives in the known arts of conveyor belt technology were evaluated in efforts to achieve criteria (a), (b), (c), and (d) above for efficiently processing wet nonwoven fiber batts through high expression squeeze rolls at liquor expression rates ranging from about 40 to 280 pounds of treating liquor per minute, equal to about 4.8 to 33.5 gallons per minute from cotton fiber batts measuring 42 inches wide, weighing from about 12 ounces per square yard to about 32 ounces per square yard. However, none of the existing known prior art systems were satisfactory for achieving the combined criteria (a), (b), (c) and (d) noted above. Some of the reasons for the inadequacy of known prior art conveyor belt systems are discussed below.
First, in order to meet criterion (b), the conveyor belt must be sufficiently porous to pass a large portion of the liquor expressed from the batt through the belt. To be satisfactory, the liquor from the batt must pass through porous openings in the conveyor belt in a path normal to the face of the belt fabric by reason of the pressure exerted on the batt by the belt and the upper squeeze roll (just prior to the entry of the belt and the batt into the nip of the paired high expression nip rolls). A solid-non porous belt is unsatisfactory since all of the liquor so expressed must flow in a generally horizontal and disruptive flow direction more or less parallel to the axes of the squeeze rolls, and outwardly from the center of the fabric toward the selvedges of the batt. Consquently, the total mass of liquor building up in and around the batt at the nip causes frequent distortions and ruptures in the batt as the liquor is blocked by a nonporous belt from passage through the batt in the preferred path normal to the face of the batt.
Second, the pore spaces within a porous belt fabric fill with a portion of the rinsing or treating liquor which is expressed from the batt at the squeeze roll nip. Also, the pore spaces or voids between fibers of the batt are fully saturated with liquor, but become relatively small in volume, roughly on the order of 0.40 to 0.60 fractional volume of the total volume occupied by the fiber plus the liquid, in the wet compressed batt in the area of the nip between the squeeze rolls. Since many cotton fabrics and nonwoven batts contain an abundance of very fine capillary pore systems within and between the cotton fibers, and since fine capillaries are more highly competitive than coarse capillaries, the fine capillaries present in the cotton fabrics will draw or "rob" liquid from the coarser capillary void spaces which characterize most wire or plastic woven conveyor belts.
Translation of the volume density of water, for example, to various area density values as a function of film thickness is very enlightening in understanding the need for avoiding excessive pore volume capacity of the conveyor belt which passes through the nip of the squeeze rolls. A film of water at a density of 1.0 gram per cubic centimeter will weigh 0.0468 pound per square yard for each 1.0 mil of film thickness. Since 1/16-inch equals 0.0625 inch or 62.5 mils of thickness, a 1/16-inch thick water film will weigh 2.925 pounds per square yard, and corresponds to a wet pickup of 292.5% on the weight of a 16-ounce dry fiber per square yard batt, abbreviated as 292.5% OWF.
A sturdy woven wire conveyor belt can easily carry the equivalent of a 1/16-inch thick film of water within the interstices of the wire belt. Hence the practice of conveying a medium weight (16 oz/sq yd) nonwoven cotton batt between the nip of a pair of high expression squeeze rolls can reduce the aqueous liquor content of a 16 oz/square yard cotton batt down to roughly 80% wet pick-up providing that the cotton batt is passed through the nip of the squeeze rolls without the conveyor belt passing through the nip. However, the equivalency of a 1/16-inch thick water film which would also pass through the nip entrained in such a wire conveyor belt would carry an additional theoretical 292% OWF liquor through the nip rolls to be reabsorbed by the cotton batt immediately downstream of the nip.
Furthermore, experimentally measured data for scoured and bleached cotton fiber batts illustrate the point. Such fiber batts may carry on the order of 10 pounds or more of rinse water per pound (dry basis) of cotton fiber as the wet fiber batt is transported from the rinser to the paired high expression squeeze rolls. If this wet fiber batt passes directly into the nip between the squeeze rolls, without the aid of an auxiliary transfer conveyor belt, the water content is typically reduced to some level of residual wet pickup on the order of 0.8 to 1.3 pounds of liquor per pound of fiber. Using density values of 1.54 grams per cubic centimeter for cellulose and 1.0 grams per cubic centimeter for water, the fractional component volumes of air, water and cellulose fiber in the web cotton batt discharged from the nip of the paired high expression squeeze rolls may be calculated on the basis of the measured wet and dry batt area density values and the thickness of the wet batt. For example, typical values for component fractional volumes are on the order of 0.10 for the dry cellulose of the cotton fiber, 0.20 for the water content in the wet cotton batt, and 0.70 for the fraction volume of air present due to the expansion of the fiber batt after leaving the high compression nip. The 0.10 volume fraction at a density of 1.54 gram per cubic centimeter corresponds to 0.154 gram for the cellulose of cotton fiber. The 0.20 volume fraction of water at a density of 1.0 gram per cubic centimeter corresponds to 0.20 gram of water, equivalent to 1.30 pound of water per pound of dry fiber. If all of the remaining 0.70 volume fraction filled with air is capable of absorbing water from the saturated conveyor belt, an additional wet pickup capacity of 4.54 pounds water per pound of dry fiber is possible.
Consequently, even a conveyor belt fabric measuring only 50 mils thick and characterized by a void volume fraction of, say, 0.60 will contain approximately 1.40 pounds of water per square yard if all of the void spaces are fully saturated, i.e., filled with water. If only 50% of that liquid migrates into a cotton batt containing 16 ounces of dry fiber per square yard, the batt will reabsorb 0.70 pound of water per square yard of batt, equivalent to an increase of 70% in wet pick-up.
It is, therefore, highly desirable to reduce both the thickness and the fraction void volume of conveyor belt fabrics used to convey nonwoven batts through paired squeeze roll nips in order to reduce the total volume capacity of the belt for carrying liquid through the nip. Although tighter weave constructions will reduce fabric void volumes, it is necessary to maintain sufficient open area in the weave pattern to permit the liquors expressed at the squeeze roll to pass easily through the interstices of the fabric weave pattern normal to the plane of the fabric face. Consequently it is preferred to reduce the fabric thickness to reduce the fabric pore volume and also at the same time to reduce the resistance to fluid flow through the belt fabric to facilitate the achievement of criterion (a) for the fiber batt auxiliary transfer conveyor belt.
Thin, light weight woven fabric belts unfortunately lack the stiffness required to maintain the dimensional stability necessary for conventional belt tracking devices such as crowned rolls, belt aligning rolls, fabric edge guides or bumper guides.
Many efforts were made to discover a conveyor belt fabric which could be used to successfully convey the batt through the nip of paired high expression squeeze rolls. Those fabric designs which were considered to be sufficiently dimensionally stable to enable an endless conveyor belt to be self-guiding (or guided by means of conventional arrangements or combinations of centering rolls, crowned turn rolls, etc., well known to those skilled in fabrication and use of such devices) frequently failed to respond to such well known belt tracking arrangements. The passage of the endless transfer belt through the nip of the paired high expression squeeze rolls itself appears to contribute to the tracking problems. Also, an acceptable nip roll transfer belt must be relatively short in length to accommodate the relatively small span length distances between belt turn rolls and auxiliary guiding rolls in the space available adjacent to a conventional paired squeeze roll stand. Such short spans are preferred in the practical economic sense to minimize space requirements, since five or more high expression paired squeeze roll transfer positions are needed, for example, in a simple full scouring and bleaching continuous process for cotton staple.
It is well known that the shorter the belt, the more difficult it is to guide the motion of the belt and keep the belt from tracking off of the center of the belt turn rolls, even with the highly sophisticated automatic belt tracking devices known in the art.
A further complication in the effective employment of conventional belt guiding systems is the fact that the area density of a fiber batt may vary at times from point to point due to an occasional fold, wrinkle, or partial discontinuity in the batt which may occur from time to time in the continuous process. The dominating and controlling driving force applied to the belt is provided by the paired high expression squeeze rolls as the belt (with the superimposed batt) passes through the nip between the squeeze rolls. Consequently, this combination of circumstances may also significantly interfere with conventional belt guiding systems.
And furthermore, when either lightweight, fine textured conveyor belt fabrics, or thin gage more open mesh fabrics were employed with conventional belt guiding aids, the fabrics were more prone to skew, bow, and neck-in within a relatively short period of use. Stretching of the fabric may occur with crowned rolls, defeating the purpose of the crowned roll. If all of the belt guiding turn rolls are not in perfect alignment and true in diameter and concentricity, or if manually or automatically adjusted pivoting turn rolls or guiding rolls are used, the warp and filling yarns (normally oriented perpendicular to each other in the fabric weave pattern) begin to form skewed patterns, i.e., to lose the rectangular orientation between warp and filling yarns. In this manner, a rectangular weave pattern may shift to non-rectangular parallelograms or S-shaped weave patterns. Hence, the fabric becomes progressively narrower in width. The loss in belt fabric working width is in itself highly undesirable. And the shifting weave patterns, loss of the original rectangular belt dimensions and length to width relationships combine to overcome and render ineffective the conventional arts employed to guide endless conveyor belts.
Accordingly, it is an object of the present invention to provide a method and an apparatus for expressing liquor from nonwoven batts in a manner which will limit distortion and prevent rupturing of the batt.
It is a further object of the present invention to provide smooth and uninterrupted transfer of the fiber batt from one impregnator or rinser primary conveyor belt, as the batt passes through the nip of paired high expression squeeze rolls, to the next primary conveyor belt in a subsequent fiber treating vessel or stage.
Another object of the present invention is to provide a method and apparatus which will assure high liquor expression efficiency from the batt so as to facilitate further processing of the batt.
Yet another object of the present invention is to provide auxiliary conveyor belt systems of improved design which will convey the fiber batt through the nip of the paired squeeze rolls and which will permit a more favorable removal of expressed liquor away from the batt than is possible with the presently known conveyor belts and associated guiding devices.
These and other objects of the present invention are realized in various embodiments by utilizing preferred auxiliary transfer conveyor belt fabric designs and guiding devices in conjunction with a pair of high expression squeeze rolls to minimize distortion and rupturing of the batt while maintaining high squeeze roll liquor expression efficiencies.
According to a preferred embodiment of the present invention, the pair of high expression squeeze rolls are arranged with their axes oriented horizontally in a vertical plane and with an auxiliary transfer conveyor belt of suitable fabric design and suitable belt guiding means arranged so as to squeeze the batt at a point along the circumference of the upper squeeze roll significantly above a horizontal plane passing through the nip of the paired high expression squeeze rolls, and then to convey the batt through the nip of the paired high expression squeeze rolls.
According to another preferred embodiment of the present invention, the auxiliary transfer conveyor belt is provided with a pair of guiding chains connected to the belt along the selvedges of the belt. Various sprockets and grooved pulleys, in turn, guide the chains and accordingly align the conveyor belt through the nip and over the various turn rolls.
If desired, a pair of sprockets locked to a common shaft may also be utilized to maintain a preferred alignment of the belt and chains. A torque assist may be provided such as a pair of sprockets (locked to a common shaft) to selectively advance both of the guiding chains simultaneously relative to the belt. Various tensioning mechanisms may be provided to tension either the belt, both chains or selectively only one or the other chain as desired.